Sensorineural hearing impairment is characterized not only by a reduction in auditory sensitivity, but also by a loss in frequency selectivity due to abnormally wide auditory filters in the damaged cochlea. The possible functional consequences of such a deficit include an inability to adequately analyze the spectral detail of complex sounds, an unfavorable signal-to-noise ratio at the outputs of the auditory filters, and abnormal temporal interactions among adjacent spectral components. These factors may combine to interfere with the understanding of speech, particularly in noisy environments. The long-term goals of this series of experiments are to more fully describe the characteristics of reduced frequency selectivity associated with various degrees of sensitivity loss and with the normal aging process, and to assess the relationship between auditory filter bandwidths and the ability to identify and discriminate broadband harmonic complexes. Frequency selectivity will be assessed in hearing-impaired listeners using a notched noise masking method to derive the shape and bandwidths of auditory filters. Measures of the internal spectral representation of complex sounds, identification of harmonic complexes, and discrimination of phase changes among the components of harmonic complexes will be evaluated in terms of the magnitudes of the filter bandwidths. In addition, experiments are proposed which assess both the accuracy of modeling internal spectral representations with measures of frequency selectivity, and the value of simulating the effects of hearing impairment with background noise.